A cathode for hydrogen generation has been used in electrolysis in which water or an aqueous solution of an alkali metal compound (typically an alkali metal chloride) is electrolyzed to produce hydrogen, chlorine, caustic soda, and the like. The major problem in electrolysis is reduction of energy consumption, more specifically, reduction in electrolytic voltage. In recent years, as an electrolytic process for an aqueous solution of an alkali metal chloride such as salt water, an ion-exchange membrane process is common, and various studies have been carried out now. When electrolysis is carried out, as an electrolytic voltage, in addition to a voltage theoretically required for electrolysis of salt, an overvoltage due to anodic reaction (generation of chlorine), an overvoltage due to cathodic reaction (generation of hydrogen), a voltage by resistance of an ion-exchange membrane, and a voltage depending on an interelectrode distance between anode and cathode, are required. Among these voltages, regarding an overvoltage due to electrode reactions, as an anode for chlorine generation, a noble-metal-based electrode, so-called DSA (Dimensionally Stable Anode), has been developed, in which chlorine overvoltage has been greatly reduced even to 50 mV or lower. On the other hand, regarding a cathode associated with hydrogen generation, in recent years, a durable cathode having low hydrogen overvoltage has been demanded from the viewpoint of energy saving. In addition, it is known that when operation of an electrolyzer is stopped, the cathode is exposed to an oxidative atmosphere by the reverse current, and resistance to the oxidative deterioration due to this reverse current has been also demanded. In order to prevent the oxidative deterioration of the cathode, a step of passing weak protection current before stopping the operation of electrolyzer is employed. However, this method of stopping the operation of electrolyzer needs to be improved due to complicated operational procedures and cost increase in ancillary facilities, and the like. Therefore, a cathode, which can be stopped without passing protection current in stopping the operation of electrolyzer, has been demanded.
As a cathode for hydrogen generation, soft steel, stainless steel and nickel has been used, and activation of the surface of these metals to reduce hydrogen overvoltage has been studied, and many patent applications filed. A typical catalyst layer of a hydrogen generation cathode includes nickel, nickel oxide, nickel-tin alloy, a combination of activated charcoal and oxides, ruthenium oxide, platinum, and the like. In addition, a method for producing a cathode for hydrogen generation may include alloy plating, dispersion/composite plating, thermal decomposition, thermal spraying, and combinations thereof, and the like.
A cathode for hydrogen generation, in which a nickel oxide layer has been formed on a nickel base material by plasma spraying fine particles of granulated nickel oxide, has been developed and used (Non-Patent Document 1). This cathode has a feature that is very resistant to the oxidative deterioration due to electric current because the catalyst itself is an oxide, and does not require a protection current when operation of the electrolyzer is stopped.
As described in Non-Patent Document 2, dispersion plating in which Raney nickel and a hydrogen storing alloy are combined has been used. Raney nickel can realize a low hydrogen overvoltage because it has a very large effective area. Though Raney nickel has an oxidation-labile property, preventing oxidation caused by the reverse current generated when operation of the electrolyzer is stopped by introducing the hydrogen storing alloy has been carried out.
As a cathode using a noble metal, a cathode composed of ruthenium oxide has been proposed, which has a very low hydrogen overvoltage as a cathode for hydrogen generation in an aqueous solution of an alkali metal. However, it is known that ruthenium oxide is subjected to an oxidative degradation by reverse current, and therefore, it is necessary that the protection current is passed when operation of the electrolyzer is stopped.
Patent Document 1 describes that durability of an electrode can be improved by forming an electrode catalyst layer including mainly ruthenium on a metal base material, and further forming a porous protective layer having low activity on the surface thereof.
Forming an electrode catalyst layer having a coating composed of ruthenium oxide, nickel and a rare earth metal having hydrogen storing ability, which was formed by thermal decomposition method, has been also proposed. By introducing the hydrogen storing alloy, preventing oxidation caused by the reverse current generated when operation of the electrolysis is stopped (Patent Document 2).
Since platinum is an electrochemically stable material having a low hydrogen overvoltage, a cathode having a low hydrogen overvoltage by supporting platinum in the catalyst layer has been proposed. However, a cathode for hydrogen generation using only platinum has a problem in durability because platinum physically drops off during electrolysis. Further, it is also a serious problem that the cathode is easily poisoned by Fe ion included in the electrolytic solution leading to a rise in electrolytic voltage.
In Patent Document 3, a cathode for hydrogen generation composed of platinum and cerium oxide has been proposed. In Patent Document 4, a cathode for hydrogen generation composed of platinum-nickel alloy has been proposed. Both of these cathodes exhibit superior performances as a cathode for hydrogen generation in an aqueous solution of alkali metal, but further studies are being carried out in order to improve on the cost.
In Patent Document 5, a cathode for hydrogen generation composed of platinum and iridium oxide has been proposed. However, because of a low degree in crystallinity of iridium oxide and insufficient durability against reverse current, this cathode for hydrogen generation has not been industrialized.
As mentioned above, many approaches have been studied, and various cathodes for hydrogen generation have been proposed for the purpose of reducing power consumption. However, a cathode for hydrogen generation having a low hydrogen overvoltage and sufficient durability against the reverse current and Fe impurities in the electrolytic solution, and further resistance against the reverse current when electrolysis is stopped, has not yet been realized.